Diagenetic Characteristics and Pore Evolution of Low-Permeability Sandstone Reservoirs: A Case Study of Chang 2 Reservoir in Jiyuan Area, Ordos Basin

The present study conducted research on diagenesis, lithofacies, pore evolution, and relatively high-permeability genesis using thin sections of cast bodies, scanning electron microscopy, cathodoluminescence (CL), X-ray diffraction, physical properties, burial history, and diagenetic evolution. Based on the controlling factors of reservoir current appearance and pore evolution, lithofacies were classified. Based on the reservoir petrologic characteristics, facial rate, and microscopic data, combined with the microcosmic and macroscopic response relationship between sedimentary and diagenetic mineral characteristics, pore types, and reservoir diversity changes, the present features such as mineral, pore structure, and physical properties of the dominant sand body are highlighted. Through pore evolution simulation, the unconsolidated initial porosity of the reservoir is restored (49.55%). The porosity changes after compaction (18.41%), early cementation-metasomatism (9.77%), and dissolution-middle and late cementation (14.93%); apparent compaction rate, apparent cementation rate, and apparent dissolution rate were used to quantify the degree of pore space construction and destruction by diagenesis. In the process of diagenetic facies division, quantitative evaluation of diagenetic intensity is introduced to accurately classify the good and bad reservoir and screen out the reservoir and seepage facies with a high-quality coefficient. There are four diagenetic phases: low-permeability-medium-pore weakly cemented dissolution facies, ultralow-permeability-low-pore medium-compacted medium-cemented dissolution facies, ultralow-permeability-low-pore strong-compacted cemented facies, and ultralow-permeability-ultralow-pore strongly compacted carbonate-cemented facies.